高分辨率成像揭示了地下深处令人费解的特征

由剑桥大学领导的新研究首次获得了位于地核边界层中一个异常岩石袋的详细“图像”，该边界层位于地表以下约 3000 公里处。

几乎位于夏威夷群岛正下方的神秘岩石区域是几个速度极低的区域之一 – 之所以这么称呼是因为地震波在穿过它们时会慢到爬行。

2022 年 5 月 19 日在杂志上发表的研究 自然通讯，是第一个详细揭示这些口袋之一的复杂内部各向异性的人，揭示了地球深处的景观以及在其中运行的过程。

“在地球的所有深层内部特征中，这是最迷人和最复杂的。我们现在已经获得了第一个确凿的证据来证明它的内部结构——它是深层地震学的真正里程碑，”主要作者、博士生李志说在剑桥地球科学系。地面”。

地球的内部像洋葱一样形成：中心是铁镍核心，被称为地幔的厚层包围，在其上方是薄薄的外壳——我们赖以生存的地壳。 虽然地幔是一块坚硬的岩石，但它足够热，流动得非常缓慢。 内部对流将热量输送到地表，导致构造板块运动并助长火山喷发。

科学家们利用地震产生的地震波“看到”地球表面之下的东西——这些波的回声和阴影揭示了内部深处的雷达图像。 但是，直到最近，核心-地幔边界的结构“图像”是研究我们星球内部热流的主要兴趣区域，它是颗粒状的，难以解释。

本研究中使用的 Sdiff 射线的事件和轨迹。 a) 穿过夏威夷超低速区域中心的横截面，显示了一维 PREM 地球模型在 96°、100°、110° 和 120° 处 Sdiff 波的射线轨迹。 从上到下的虚线表示 410 公里、660 公里和 2,791 公里的不连续面（核幔边界以上 100 公里）。 b) 背景断层扫描模型 SEMUCB_WM1 在 2791 公里深度的事件和 Sdiff 射线轨迹。 用于活动的沙滩球涂有不同颜色，包括 20100,320（黄色）、20111214（绿色）、20120417（红色）、20180910（紫色）、20180518（棕色）、20181030（粉色）、20161122（灰色）、车站（三角形）和射线。 本研究使用的下地幔孔深 2791 km 处的 Sdiff 波轨迹。 短期分析中使用的事件以黄色突出显示。 建议的 ULVZ 位置以黑色圆圈显示。 虚线显示在 A 中绘制的横截面。来源：Nature Communications，DOI：10.1038/s41467-022-30502-5

研究人员使用最先进的数值建模方法来检测核幔边界处的千米级结构。 据合著者冷匡戴博士说，他在大学期间开发了这些方法。[{” attribute=””>University of Oxford, “We are really pushing the limits of modern high-performance computing for elastodynamic simulations, taking advantage of wave symmetries unnoticed or unused before.” Leng, who is currently based at the Science and Technology Facilities Council, says that this means they can improve the resolution of the images by an order of magnitude compared to previous work.

The researchers observed a 40% reduction in the speed of seismic waves traveling at the base of the ultra-low velocity zone beneath Hawaii. This supports existing proposals that the zone contains much more iron than the surrounding rocks – meaning it is denser and more sluggish. “It’s possible that this iron-rich material is a remnant of ancient rocks from Earth’s early history or even that iron might be leaking from the core by an unknown means,” said project lead Dr Sanne Cottaar from Cambridge Earth Sciences.

Conceptual cartoons of the Hawaiian ultra-low velocity zone (ULVZ) structure. A) ULVZ on the core–mantle boundary at the base of the Hawaiian plume (height is not to scale). B) a zoom in of the modeled ULVZ structure, showing interpreted trapped postcursor waves (note that the waves analyzed have horizontal displacement). Credit: Nature Communications, DOI: 10.1038/s41467-022-30502-5

The research could also help scientists understand what sits beneath and gives rise to volcanic chains like the Hawaiian Islands. Scientists have started to notice a correlation between the location of the descriptively-named hotspot volcanoes, which include Hawaii and Iceland, and the ultra-low velocity zones at the base of the mantle. The origin of hotspot volcanoes has been debated, but the most popular theory suggests that plume-like structures bring hot mantle material all the way from the core-mantle boundary to the surface.

With images of the ultra-low velocity zone beneath Hawaii now in hand, the team can also gather rare physical evidence from what is likely the root of the plume feeding Hawaii. Their observation of dense, iron-rich rock beneath Hawaii would support surface observations. “Basalts erupting from Hawaii have anomalous isotope signatures which could either point to either an early-Earth origin or core leaking, it means some of this dense material piled up at the base must be dragged to the surface,” said Cottaar.

More of the core-mantle boundary now needs to be imaged to understand if all surface hotspots have a pocket of dense material at the base. Where and how the core-mantle boundary can be targeted does depend on where earthquakes occur, and where seismometers are installed to record the waves.

The team’s observations add to a growing body of evidence that Earth’s deep interior is just as variable as its surface. “These low-velocity zones are one of the most intricate features we see at extreme depths – if we expand our search, we are likely to see ever-increasing levels of complexity, both structural and chemical, at the core-mantle boundary,” said Li.

They now plan to apply their techniques to enhance the resolution of imaging of other pockets at the core-mantle boundary, as well as mapping new zones. Eventually, they hope to map the geological landscape across the core-mantle boundary and understand its relationship with the dynamics and evolutionary history of our planet.

Reference: “Kilometer-scale structure on the core–mantle boundary near Hawaii” by Zhi Li, Kuangdai Leng, Jennifer Jenkins and Sanne Cottaar, 19 May 2022, Nature Communications.
DOI: 10.1038/s41467-022-30502-5